Ripple-Current-Charging Strategies for LLC Resonant Converters with Enhanced Frequency-Modulated Control

This paper proposes a LLC resonant converter for battery charging applications. In order to increase the charging efficiency and extend the battery life, three charging methods, including the constant current (CC) charging, the pulse-ripple-current (PRC) charging and the sinusoidal-ripple-current (SRC) charging are adopted and investigated. Besides, an enhanced frequency-modulated control (EFMC) is developed for the proposed charger. With the proposed EFMC, the switching frequency will be dynamically regulated according to different battery voltages and load conditions. In addition, because of the zero-voltage switching (ZVS) or the zero-current switching (ZCS) feature, the proposed LLC resonant converter can reduce the circuit losses and increase the charging efficiency. The control strategies will be realized by the digital signal processor (DSP) without adding extra circuit. Operational principles and theoretical analysis will also be presented. Finally, a 720W prototype circuit will be implemented for the 48V battery system. Both simulation and experimental results verify the performance and feasibility of the proposed LLC charger.